Thermoplastic Prepreg Containing Continuous and Long Fibers

ABSTRACT

A prepreg that contains a plurality of unidirectionally aligned continuous fibers embedded within a thermoplastic polymer matrix is provided. In addition to continuous fibers, the prepreg also contains a plurality of long fibers that are combined with the continuous fibers so that they are randomly distributed within the thermoplastic matrix. As a result, at least a portion of the long fibers become oriented at an angle (e.g., perpendicular) relative to the direction of the continuous fibers. Through such orientation, the long fibers can substantially increase the mechanical properties of the prepreg in the transverse direction (e.g., strength) and thus achieve a more isotropic material. Although unique isotropic prepregs are one aspect of the present invention, it should be understood that this is not a requirement. In fact, one notable feature of the present invention is the ability to tailor the mechanical properties of the prepreg for an intended application by selectively controlling certain process parameters, such as the type of long fibers employed, the type of continuous fibers employed, the concentration of the long fibers, the concentration of the continuous fibers, the thermoplastic resin(s) employed, etc.

RELATED APPLICATIONS

The present application claims priority to Provisional Application Ser.No. 61/357,301, filed on Jun. 22, 2010, the entire contents of which areincorporated herein by reference thereto.

BACKGROUND OF THE INVENTION

Reinforced composite materials (or “prepregs”) have conventionally beenformed from fibers (e.g., carbon or glass fibers) that are impregnatedwithin a resinous matrix. Thermoset resins (e.g., unsaturated polyester,epoxy, or polyimide) are often employed as the matrix, particularly inapplications requiring a high level of strength. One problem withprepregs formed from thermoset resins, however, is that they aregenerally brittle and have poor impact resistance. Furthermore, theprepregs are often difficult to store due to the short shelf life of theresin. In an attempt to overcome these issues, efforts have recentlybeen made develop thermoplastic prepregs. One such prepreg is formedfrom a thermoplastic resin and unidirectionally aligned continuousfibers. Such a prepreg has excellent performance in terms of modulus ofelasticity and strength in the fiber axis direction. However, becausesuch prepregs have anisotropic mechanical properties, multiple prepreglayers are required during use that are oriented in differentdirection(s). This inevitably causes an increase in the cost andthickness of the resulting part. Other attempts to solve the problemassociated with thermoset prepregs involve the use of a thermoplasticresin and chopped fibers formed by cutting unidirectionally alignedstrands. While such prepregs exhibit better isotropic strengthproperties, the maximum volume fraction of the reinforcing fiber isgenerally low, which results in relatively poor modulus of elasticityand strength. Furthermore, it is difficult to tailor the mechanicalproperties of the prepreg through manipulation of the volume of suchchopped fibers.

As such, a need currently exists for a method of forming thermoplasticprepregs that allows for selective control of its mechanical propertiesdepending on the particular application. A need also exists forthermoplastic prepregs that have isotropic mechanical properties.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, athermoplastic prepreg is disclosed that comprises a plurality ofcontinuous fibers that are substantially oriented in a longitudinaldirection and a plurality of randomly distributed long fibers, at leasta portion of which are oriented at an angle relative to the longitudinaldirection. The continuous fibers constitute from about 10 wt. % to about80 wt. % of the prepreg and the long fibers constitute from about 2 wt.% to about 35 wt. % of the prepreg. The prepreg also comprises aresinous matrix that contains one or more thermoplastic polymers andwithin which the continuous fibers and long fibers are embedded, whereinthe thermoplastic polymers constitute from about 10 wt. % to about 80wt. % of the prepreg. The ratio of the maximum tensile stress of theprepreg in the longitudinal direction to the maximum tensile stress ofthe prepreg in the transverse direction is from about 1 to about 40.

In accordance with another embodiment of the present invention, a methodfor forming a thermoplastic prepreg is disclosed. The method comprisessupplying continuous fibers and long fibers to an extrusion device andsupplying a thermoplastic feedstock to the extrusion device, wherein thefeedstock comprises at least one thermoplastic polymer. The continuousfibers, long fibers, and the thermoplastic polymer are extruded withinan impregnation die to form an extrudate in which the continuous fibersare intermixed with the long fibers and embedded with a matrix of thethermoplastic polymer.

In accordance with yet another embodiment of the present invention, amethod for forming a thermoplastic prepreg is disclosed. The methodcomprises supplying continuous fibers and a thermoplastic feedstock tothe extrusion device, wherein the feedstock comprises at least onethermoplastic polymer. The continuous fibers and feedstock are extrudedwithin an impregnation die to form an extrudate that in which thecontinuous fibers are embedded with a matrix of the thermoplasticpolymer. Thereafter, long fibers are applied to the extrudate to form acomposite.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying figures, in which:

FIG. 1 is a schematic illustration of one embodiment of an impregnationsystem for use in the present invention;

FIG. 2A is a cross-sectional view of the impregnation die shown in FIG.1;

FIG. 2B is an exploded view of one embodiment of a manifold assembly andgate passage for an impregnation die that may be employed in the presentinvention;

FIG. 2C is a perspective view of one embodiment of a plate at leastpartially defining an impregnation zone that may be employed in thepresent invention;

FIG. 3 is a transverse cross-sectional view of one embodiment of aprepreg made with the system of FIG. 1;

FIG. 4 is a longitudinal cross-sectional view of the prepreg of shown inFIG. 3;

FIG. 5 is a schematic illustration of one embodiment of a pultrusionsystem that may be employed in forming a profile from the prepreg of thepresent invention;

FIG. 6 is a transverse cross-sectional view of anode embodiment of aprepreg made of the present invention;

FIG. 7 is a longitudinal cross-sectional view of the prepreg of shown inFIG. 6;

FIG. 8 is a schematic illustration of yet another embodiment of animpregnation system for use in forming the prepreg the presentinvention;

FIG. 9 is a transverse cross-sectional view of one embodiment of aprepreg made with the system of FIG. 8;

FIG. 10 is a longitudinal cross-sectional view of the prepreg of shownin FIG. 9; and

FIG. 11 is a cross-sectional view of one embodiment of a profile thatmay be formed in accordance with the present invention.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

Generally speaking, the present invention is directed to a prepreg thatcontains a plurality of unidirectionally aligned continuous fibersembedded within a thermoplastic polymer matrix. In addition tocontinuous fibers, the prepreg also contains a plurality of long fibersthat are combined with the continuous fibers so that they are randomlydistributed within the thermoplastic matrix. As a result, at least aportion of the long fibers become oriented at an angle (e.g.,perpendicular) relative to the direction of the continuous fibers.Through such orientation, the long fibers can substantially increase themechanical properties of the prepreg in the transverse direction (e.g.,strength) and thus achieve a more isotropic material. Although uniqueisotropic prepregs are one aspect of the present invention, it should beunderstood that this is not a requirement. In fact, one notable featureof the present invention is the ability to tailor the mechanicalproperties of the prepreg for an intended application by selectivelycontrolling certain process parameters, such as the type of long fibersemployed, the type of continuous fibers employed, the concentration ofthe long fibers, the concentration of the continuous fibers, thethermoplastic resin(s) employed, etc.

Various embodiments of the present invention will now be described inmore detail.

I. Continuous Fibers

The term “continuous fibers” refers to fibers, filaments, yarns, orrovings (e.g., bundles of fibers) having a length that is generallylimited only by the length of the part. For example, such fibers mayhave a length greater than about 25 millimeters, in some embodimentsabout 50 millimeters or more, and in some embodiments, about 100millimeters or more. The continuous fibers may be formed from anyconventional material known in the art, such as metal fibers; glassfibers (e.g., E-glass, A-glass, C-glass, D-glass, AR-glass, R-glass,S1-glass, S2-glass), carbon fibers (e.g., graphite), boron fibers,ceramic fibers (e.g., alumina or silica), aramid fibers (e.g., Kevlar®marketed by E. I. duPont de Nemours, Wilmington, Del.), syntheticorganic fibers (e.g., polyamide, polyethylene, paraphenylene,terephthalamide, polyethylene terephthalate and polyphenylene sulfide),and various other natural or synthetic inorganic or organic fibrousmaterials known for reinforcing thermoplastic compositions. Glass fibersand carbon fibers are particularly desirable for use in the continuousfibers. Such fibers often have a nominal diameter of about 4 to about 35micrometers, and in some embodiments, from about 9 to about 35micrometers. The fibers may be twisted or straight. If desired, thefibers may be in the form of rovings (e.g., bundle of fibers) thatcontain a single fiber type or different types of fibers. Differentfibers may be contained in individual rovings or, alternatively, eachroving may contain a different fiber type. For example, in oneembodiment, certain rovings may contain continuous carbon fibers, whileother rovings may contain glass fibers. The number of fibers containedin each roving can be constant or vary from roving to roving. Typically,a roving may contain from about 1,000 fibers to about 50,000 individualfibers, and in some embodiments, from about 2,000 to about 40,000fibers.

II. Long Fibers

The term “long fibers” generally refers to fibers, filaments, yarns, orrovings that are not continuous and have a length of from about 0.5 toabout 25 millimeters, in some embodiments, from about 0.8 to about 15millimeters, and in some embodiments, from about 1 to about 12millimeters. The long fibers may be formed from any of the material,shape, and/or size as described above with respect to the continuousfibers. Glass fibers and carbon fibers are particularly desirable foruse as the long fibers.

III. Thermoplastic Matrix

Any of a variety of thermoplastic polymers may be employed to form thethermoplastic matrix in which the continuous and long fibers areembedded. Suitable thermoplastic polymers for use in the presentinvention may include, for instance, polyolefins (e.g., polypropylene,propylene-ethylene copolymers, etc.), polyesters (e.g., polybutyleneterephalate (“PBT”)), polycarbonates, polyamides (e.g., Nylon™),polyether ketones (e.g., polyetherether ketone (“PEEK”)),polyetherimides, polyarylene ketones (e.g., polyphenylene diketone(“PPDK”)), liquid crystal polymers, polyarylene sulfides (e.g.,polyphenylene sulfide (“PPS”)), fluoropolymers (e.g.,polytetrafluoroethylene-perfluoromethylvinylether polymer,perfluoro-alkoxyalkane polymer, petrafluoroethylene polymer,ethylene-tetrafluoroethylene polymer, etc.), polyacetals, polyurethanes,polycarbonates, styrenic polymers (e.g., acrylonitrile butadiene styrene(“ABS”)), and so forth. ABS is a particularly suitable thermoplasticpolymer.

One or multiple thermoplastic matrices may be employed in the prepreg.For example, in one embodiment, the long fibers are initiallypre-impregnated with a first thermoplastic matrix, such as in a mannerthat will be described below, and thereafter cooled and chopped into topellets having a length of about 25 millimeters or less. These pelletsmay be subsequently combined with the continuous fibers as they areimpregnated with a second thermoplastic matrix. Thus, in thisembodiment, the long fibers are actually embedded within twothermoplastic matrices. The first thermoplastic matrix employed for thelong fibers may be the same or different than the second thermoplasticmatrix. Alternatively, the first thermoplastic plastic matrix used topre-impregnate the long fibers may constitute the only thermoplasticmatrix present in the prepreg. In still another embodiment, the longfibers are not pre-impregnated and are simply combined with thecontinuous fibers as they both are impregnated with the thermoplasticmatrix.

IV. Prepreq Formation

As stated above, the prepreg is generally formed in a manner that iscapable of randomly distributing the long fibers. This may beaccomplished in a variety of ways. In one particular embodiment, forexample, long fiber thermoplastic pellets may be supplied to the hopperof an extrusion device and thereafter melt blended with continuousfibers. The pressure and force of the extrusion process cause thedesired random orientation of the long fibers within the resultingprepreg. Referring to FIG. 1, for example, one embodiment of such anextrusion device is shown. More particularly, the apparatus includes anextruder 120 containing a screw shaft 124 mounted inside a barrel 122. Aheater 130 (e.g., electrical resistance heater) is mounted outside thebarrel 122. During use, a thermoplastic polymer feedstock 127 issupplied to the extruder 120 through a hopper 126. In this particularembodiment, the feedstock 127 also contains long fibers. The long fibersmay, for example, constitute from about 5 wt. % to about 60 wt. %, insome embodiments from about 10 wt. % to about 50 wt. %, and in someembodiments, from about 20 wt. % to about 40 wt. % of the feedstock.Alternatively, the feedstock 127 may be free of long fibers, and suchfibers may be supplied at another location (not shown), such asdownstream from the hopper 126 and/or other feed ports.

Regardless, the thermoplastic feedstock 127 is conveyed inside thebarrel 122 by the screw shaft 124 and heated by frictional forces insidethe barrel 122 and by the heater 130. Upon being heated, the feedstock127 exits the barrel 122 through a barrel flange 128 and enters a dieflange 132 of an impregnation die 150. A continuous fiber roving 142 ora plurality of continuous fiber rovings 142 are supplied from a reel orreels 144 to die 150. The rovings 142 are generally kept apart a certaindistance before impregnation, such as at least about 4 millimeters, andin some embodiments, at least about 5 millimeters. The feedstock 127 mayfurther be heated inside the die by heaters 133 mounted in or around thedie 150. The die is generally operated at temperatures that aresufficient to cause melting and impregnation of the thermoplasticpolymer. Typically, the operation temperatures of the die is higher thanthe melt temperature of the thermoplastic polymer, such as attemperatures from about 200° C. to about 450° C. When processed in thismanner, the continuous fiber rovings 142 become embedded in the polymermatrix, which may be a resin 214 (FIG. 2A) processed from the feedstock127, and intermixed with the long fibers contained therein. The mixtureis then extruded from the impregnation die 150 to create an extrudate152.

A pressure sensor 137 (FIG. 2A) senses the pressure near theimpregnation die 150 to allow control to be exerted over the rate ofextrusion by controlling the rotational speed of the screw shaft 124, orthe federate of the feeder. That is, the pressure sensor 137 ispositioned near the impregnation die 150 so that the extruder 120 can beoperated to deliver a correct amount of resin 214 for interaction withthe fiber rovings 142. After leaving the impregnation die 150, theextrudate 152, or impregnated fiber rovings 142, may enter an optionalpre-shaping, or guiding section (not shown) before entering a nip formedbetween two adjacent rollers 190. Although optional, the rollers 190 canhelp to consolidate the extrudate 152 into the form of a ribbon (ortape), as well as enhance fiber impregnation and squeeze out any excessvoids. In addition to the rollers 190, other shaping devices may also beemployed, such as a die system. The resulting consolidated ribbon 156 ispulled by tracks 162 and 164 mounted on rollers. The tracks 162 and 164also pull the extrudate 152 from the impregnation die 150 and throughthe rollers 190. If desired, the consolidated ribbon 156 may be wound upat a section 171. Generally speaking, the ribbons are relatively thinand typically have a thickness of from about 0.05 to about 1 millimeter,in some embodiments from about 0.1 to about 0.8 millimeters, and in someembodiments, from about 0.2 to about 0.4 millimeters.

Within the impregnation die, it is generally desired that the rovings142 are traversed through an impregnation zone 250 to impregnate therovings with the polymer resin 214. In the impregnation zone 250, thepolymer resin may be forced generally transversely through the rovingsby shear and pressure created in the impregnation zone 250, whichsignificantly enhances the degree of impregnation. This is particularlyuseful when forming a composite from ribbons of a high fiber content,such as about 35% weight fraction (“Wf”) or more, and in someembodiments, from about 40% Wf or more. Typically, the die 150 willinclude a plurality of contact surfaces 252, such as for example atleast 2, at least 3, from 4 to 7, from 2 to 20, from 2 to 30, from 2 to40, from 2 to 50, or more contact surfaces 252, to create a sufficientdegree of penetration and pressure on the rovings 142. Although theirparticular form may vary, the contact surfaces 252 typically possess acurvilinear surface, such as a curved lobe, rod, etc. The contactsurfaces 252 are also typically made of a metal material.

FIG. 2A shows a cross-sectional view of an impregnation die 150. Asshown, the impregnation die 150 includes a manifold assembly 220, a gatepassage 270, and an impregnation zone 250. The manifold assembly 220 isprovided for flowing the polymer resin 214 therethrough. For example,the manifold assembly 220 may include a channel 222 or a plurality ofchannels 222. The resin 214 provided to the impregnation die 150 mayflow through the channels 222.

As shown in FIG. 2B, some portions of the channels 222 may becurvilinear, and in exemplary embodiments, the channels 222 have asymmetrical orientation along a central axis 224. Further, in someembodiments, the channels may be a plurality of branched runners 222,which may include first branched runner group 232, second group 234,third group 236, and, if desired, more branched runner groups. Eachgroup may include 2, 3, 4 or more runners 222 branching off from runners222 in the preceding group, or from an initial channel 222.

The branched runners 222 and the symmetrical orientation thereofgenerally evenly distribute the resin 214, such that the flow of resin214 exiting the manifold assembly 220 and coating the rovings 142 issubstantially uniformly distributed on the rovings 142. This desirablyallows for generally uniform impregnation of the rovings 142.

Further, the manifold assembly 220 may in some embodiments define anoutlet region 242, which generally encompasses at least a downstreamportion of the channels or runners 222 from which the resin 214 exits.In some embodiments, at least a portion of the channels or runners 222disposed in the outlet region 242 have an increasing area in a flowdirection 244 of the resin 214. The increasing area allows for diffusionand further distribution of the resin 214 as the resin 214 flows throughthe manifold assembly 220, which further allows for substantiallyuniform distribution of the resin 214 on the rovings 142.

As further illustrated in FIGS. 2A and 2B, after flowing through themanifold assembly 220, the resin 214 may flow through gate passage 270.Gate passage 270 is positioned between the manifold assembly 220 and theimpregnation zone 250, and is provided for flowing the resin 214 fromthe manifold assembly 220 such that the resin 214 coats the rovings 142.Thus, resin 214 exiting the manifold assembly 220, such as throughoutlet region 242, may enter gate passage 270 and flow therethrough, asshown.

Upon exiting the manifold assembly 220 and the gate passage 270 of thedie 150 as shown in FIG. 2A, the resin 214 contacts the rovings 142being traversed through the die 150. As discussed above, the resin 214may substantially uniformly coat the rovings 142, due to distribution ofthe resin 214 in the manifold assembly 220 and the gate passage 270.Further, in some embodiments, the resin 214 may impinge on an uppersurface of each of the rovings 142, or on a lower surface of each of therovings 142, or on both an upper and lower surface of each of therovings 142. Initial impingement on the rovings 142 provides for furtherimpregnation of the rovings 142 with the resin 214.

As shown in FIG. 2A, the coated rovings 142 are traversed in rundirection 282 through impregnation zone 250, which is configured toimpregnate the rovings 142 with the resin 214. For example, as shown inFIGS. 2A and 2C, the rovings 142 are traversed over contact surfaces 252in the impregnation zone. Impingement of the rovings 142 on the contactsurface 252 creates shear and pressure sufficient to impregnate therovings 142 with the resin 214 coating the rovings 142.

In some embodiments, as shown in FIG. 2A, the impregnation zone 250 isdefined between two spaced apart opposing plates 256 and 258. Firstplate 256 defines a first inner surface 257, while second plate 258defines a second inner surface 259. The contact surfaces 252 may bedefined on or extend from both the first and second inner surfaces 257and 259, or only one of the first and second inner surfaces 257 and 259.FIG. 2C illustrates the second plate 258 and the various contactsurfaces thereon that form at least a portion of the impregnation zone250 according to these embodiments. In exemplary embodiments, as shownin FIG. 2A, the contact surfaces 252 may be defined alternately on thefirst and second surfaces 257 and 259 such that the rovings alternatelyimpinge on contact surfaces 252 on the first and second surfaces 257 and259. Thus, the rovings 142 may pass contact surfaces 252 in a waveform,tortuous or sinusoidual-type pathway, which enhances shear.

The angle 254 at which the rovings 142 traverse the contact surfaces 252may be generally high enough to enhance shear, but not so high to causeexcessive forces that will break the fibers. Thus, for example, theangle 254 may be in the range between approximately 1° and approximately30°, and in some embodiments, between approximately 5° and approximately25°.

In alternative embodiments, the impregnation zone 250 may include aplurality of pins (not shown), each pin having a contact surface 252.The pins may be static, freely rotational, or rotationally driven. Infurther alternative embodiments, the contact surfaces 252 andimpregnation zone 250 may comprise any suitable shapes and/or structuresfor impregnating the rovings 142 with the resin 214 as desired orrequired.

To further facilitate impregnation of the rovings 142, they may also bekept under tension while present within the impregnation die. Thetension may, for example, range from about 5 to about 300 Newtons, insome embodiments from about 50 to about 250 Newtons, and in someembodiments, from about 100 to about 200 Newtons per roving 142 or towof fibers.

As shown in FIG. 2A, in some embodiments, a land zone 280 may bepositioned downstream of the impregnation zone 250 in run direction 282of the rovings 142. The rovings 142 may traverse through the land zone280 before exiting the die 150. As further shown in FIG. 2A, in someembodiments, a faceplate 290 may adjoin the impregnation zone 250.Faceplate 290 is generally configured to meter excess resin 214 from therovings 142. Thus, apertures in the faceplate 290, through which therovings 142 traverse, may be sized such that when the rovings 142 aretraversed therethrough, the size of the apertures causes excess resin214 to be removed from the rovings 142.

The impregnation die shown and described above is but one of variouspossible configurations that may be employed in the present invention.In alternative embodiments, for example, the fibers may be introducedinto a crosshead die that is positioned at an angle relative to thedirection of flow of the polymer melt. As the fibers move through thecrosshead die and reach the point where the polymer exits from anextruder barrel, the polymer is forced into contact with the fibers. Itshould also be understood that any other extruder design may also beemployed, such as a twin screw extruder. Still further, other componentsmay also be optionally employed to assist in the impregnation of thefibers. For example, a “gas jet” assembly may be employed in certainembodiments to help uniformly spread a bundle or tow of individualfibers, which may each contain up to as many as 24,000 fibers, acrossthe entire width of the merged tow. This helps achieve uniformdistribution of strength properties in the ribbon. Such an assembly mayinclude a supply of compressed air or another gas that impinges in agenerally perpendicular fashion on the moving fiber tows that passacross the exit ports. The spread fiber bundles may then be introducedinto a die for impregnation, such as described above.

Regardless of the technique employed, at least a portion of the longfibers in the prepreg are oriented at an angle relative to thelongitudinal direction in which the continuous fibers are oriented (themachine direction “A” of the system of FIG. 1). For example, about 10%or more, in some embodiments about 20% or more, and in some embodiments,about 30% or more of the fibers may be oriented at an angle relative tothe longitudinal direction of the continuous fibers. This angle may, forinstance, be about 10° to about 120°, in some embodiments from about 20°to about 110° C., and in one embodiment, about 90°. Referring to FIGS.3-4, for example, one embodiment of a prepreg 200 is shown that containsa plurality of long fibers 220 oriented at various angles relative to alongitudinal direction “L” in which continuous fibers 240 are aligned.Among other things, the angle of orientation helps control thetransverse strength of the prepreg.

In addition, the relative percentage of long fibers and continuousfibers in the prepreg also help to control the strength properties. Toachieve a good balance between tensile strength and transverse strength,the ratio of the weight of continuous fibers to the weight of longfibers is typically controlled within the range of from about 0.2 toabout 10, in some embodiments from about 0.4 to about 5, and in someembodiments, from about 0.5 to about 5. For instance, continuous fibersmay constitute from about 10 wt. % to about 80 wt. %, in someembodiments from about 20 wt. % to about 70 wt. %, and in someembodiments, from about 40 wt. % to about 60 wt. % of the prepreg.Likewise, long fibers may constitute from about 2 wt. % to about 35 wt.%, in some embodiments from about 5 wt. % to about 30 wt. %, and in someembodiments, from about 10 wt. % to about 25 wt. % of the prepreg.Thermoplastic polymer(s) may constitute from about 10 wt. % to about 80wt. %, in some embodiments from about 20 wt. % to about 70 wt. %, and insome embodiments, from about 40 wt. % to about 60 wt. % of the prepreg.

The prepreg also has a very low void fraction, which helps enhance themechanical properties of the prepreg. For instance, the void fractionmay be about 3% or less, in some embodiments about 2% or less, and insome embodiments, about 1% or less. The void fraction may be measuredusing techniques well known to those skilled in the art. For example,the void fraction may be measured using a “resin burn off” test in whichsamples are placed in an oven (e.g., at 600° C. for 3 hours) to burn outthe resin. The mass of the remaining fibers may then be measured tocalculate the weight and volume fractions. Such “burn off” testing maybe performed in accordance with ASTM D 2584-08 to determine the weightsof the fibers and the thermoplastic matrix, which may then be used tocalculate the “void fraction” based on the following equations:

V _(f)=100*(ρ_(t)−ρ_(c))/ρ_(t)

where,

V_(f) is the void fraction as a percentage;

ρ_(c) is the density of the composite as measured using knowntechniques, such as with a liquid or gas pycnometer (e.g., heliumpycnometer);

ρ_(t) is the theoretical density of the composite as is determined bythe following equation:

ρ_(t)=1/[W _(f)/ρ_(f) +W _(m)/ρ_(m)]

ρ_(m) is the density of the thermoplastic matrix (e.g., at theappropriate crystallinity);

ρ_(f) is the density of the fibers;

W_(f) is the weight fraction of the fibers; and

W_(m) is the weight fraction of the thermoplastic matrix.

Alternatively, the void fraction may be determined by chemicallydissolving the resin in accordance with ASTM D 3171-09. The “burn off”and “dissolution” methods are particularly suitable for glass fibers,which are generally resistant to melting and chemical dissolution. Inother cases, however, the void fraction may be indirectly calculatedbased on the densities of the thermoplastic polymer, fibers, and ribbonin accordance with ASTM D 2734-09 (Method A), where the densities may bedetermined ASTM D792-08 Method A. Of course, the void fraction can alsobe estimated using conventional microscopy equipment.

Through control over the various parameters mentioned above, themechanical strength properties may be tailored to the desiredapplication. In certain embodiments, for example, it is desirable thatthe prepreg exhibits relatively isotropic strength properties. Moreparticularly, for such isotropic prepregs, the ratio of the maximumtensile stress (or “ultimate strength”) in the longitudinal direction tothe maximum tensile stress in the transverse direction is typically fromabout 1 to about 40, in some embodiments from about 2 to about 30, andin some embodiments, from about 4 to about 20. In certain embodiments,the prepreg of the present invention may exhibit a maximum tensilestress in the longitudinal direction of from about 250 to about 3000Megapascals (MPa), in some embodiments from about 400 to about 2500 MPa,and in some embodiments, from about 600 to about 2000 MPa, and in thetransverse direction of from about 0.5 to about 50 MPa, in someembodiments from about 1 to about 40 MPa, and in some embodiments, fromabout 2 to about 20 MPa.

In the embodiments described above and shown in FIGS. 3-4, the longfibers are incorporated into the extrusion device during impregnationand therefore become intermingled with the continuous fibers andgenerally distributed throughout the entire prepreg. It should beunderstood, however, that such a configuration is not required. Incertain embodiments, for example, the long fibers may be combined withthe continuous fibers in such a manner that they form separate layers.The layers may be “non-discrete” in the sense that at least a portion ofthe long fibers extend into the continuous fiber layer. One embodimentof a system for forming such a non-discrete layered prepreg may includean additional device (e.g., extruder) that is employed to apply the longfibers downstream from the die for impregnating the continuous fibers.Long fibers applied in this manner are still randomly distributed withinthe prepreg, but form a separate layer. This is illustrated in moredetail in FIGS. 6-7. As shown, a layer 260 is formed that contains thelong fibers 262 and a layer 270 is formed that contains continuousfibers 272. Due in part to the manner in which they are applied, aportion 230 of the long fibers 262 may also extend into the continuousfibers layer 270.

Of course, the layers may also be “discrete” in the sense thatsubstantially all of the long fibers are contained within the layer. Oneembodiment of a system for forming such a discrete layered prepreg isshown in FIG. 8. In this particular embodiment, a pre-consolidatedcontinuous fiber prepreg 320 is unwound from a reel 330 and heated to orabove the softening point of the thermoplastic matrix within an oven 340(e.g., infrared oven). Thereafter, the softened prepreg 320 is suppliedto an extrusion device 350 where long fibers (not shown) may be applied.The resulting layered prepreg 360 may then be supplied between twoconsolidation rolls 290 as described above. Once again, the long fibersapplied in this manner are still randomly distributed within theprepreg, but form a separate “discrete” layer. This is illustrated inmore detail in FIGS. 9-10. As shown, a layer 420 is formed that containslong fibers 422 and a layer 430 is formed that contains the continuousfibers 424.

V. Articles Formed from the Prepreq

The prepreg of the present invention may generally be employed in avariety of different applications and parts. For example, the prepregmay be formed into a profile, injection molded part, compression molded,part, etc. A “profile” is hollow or solid pultruded part that maypossess a wide variety of cross-sectional shapes, such as square,rectangular, circular, elliptical, triangular, I-shaped, C-shaped,U-shaped, J-shaped, L-shaped, slotted, etc. In hollow profiles, at leasta portion of the interior of the profile is a voided space. The voidedspace may optionally extend the entire the length of the profile. Theprofiles may also be “lineal” to the extent that they possess across-sectional shape that is substantially the same along the entirelength of the profile, or they may have a varying cross-sectional shape,such as curved, twisted, etc. Regardless, such profiles may be employedas a structural member for window lineals, decking planks, railings,balusters, roofing tiles, siding, trim boards, pipe, fencing, posts,light posts, highway signage, roadside marker posts, etc.

The manner in which a profile may be formed from a prepreg can vary asis well known to those skilled in the art. One or multiple prepreglayers may be employed for forming the profile. Referring to FIG. 5, forexample, one particular embodiment of a system is shown in which aplurality of prepregs 12 are employed to form a profile. In thisembodiment, the prepregs 12 are provided in a wound package on a creel20. The creel 20 may be an unreeling creel that includes a frameprovided with horizontal rotating spindles 22, each supporting apackage. A pay-out creel may also be employed, particularly if desiredto induce a twist into the fibers. It should also be understood that theprepregs may also be formed in-line with the formation of the profile.In one embodiment, for example, the extrudate 152 exiting theimpregnation die 150 from FIG. 1 may be directly supplied to the systemused to form a profile.

A tension-regulating device 40 may also be employed to help control thedegree of tension. The device 40 may include inlet plate 30 that lies ina vertical plane parallel to the rotating spindles 22 of the creel 20.The tension-regulating device 40 may contain cylindrical bars 41arranged in a staggered configuration so that the prepregs 12 pass overand under these bars to define a wave pattern. The height of the barscan be adjusted to modify the amplitude of the wave pattern and controltension.

If desired, the prepregs 12 may be heated in an oven 45 having any of avariety of known configuration, such as an infrared oven, convectionoven, etc. During heating, the fibers are unidirectionally oriented tooptimize the exposure to the heat and maintain even heat across theentire profile. The temperature to which the ribbons 12 are heated isgenerally high enough to soften the thermoplastic polymer to an extentthat the ribbons can bond together. However, the temperature is not sohigh as to destroy the integrity of the material. The temperature may,for example, range from about 100° C. to about 300° C., in someembodiments from about 110° C. to about 275° C., and in someembodiments, from about 120° C. to about 250° C. In one particularembodiment, for example, acrylonitrile-butadiene-styrene (ABS) is usedas the polymer, and the ribbons are heated to or above the melting pointof ABS, which is about 105° C. In another embodiment, polybutyleneterephalate (PBT) is used as the polymer, and the ribbons are heated toor above the melting point of PBT, which is about 224° C.

Upon being heated, the continuous fiber ribbons 12 may be provided to aconsolidation die to help bond together different ribbon layers, as wellas for alignment and formation of the initial shape of the profile.Although referred to herein as a single die, it should be understoodthat the consolidation die 50 may in fact be formed from multipleindividual dies (e.g., face plate dies). The consolidation die 50 mayreceive the prepregs 12 so that it is guided through a channel (notshown) of the die 50 in a direction “A”. The channel may be provided inany of a variety of orientations and arrangements to result in thedesired reinforcement scheme. Within the die 50, the prepregs aregenerally maintained at a temperature at or above the melting point ofthe thermoplastic matrix used in the ribbon to ensure adequateconsolidation.

If desired, a pultrusion die 60 may also be employed that compresses theprepregs into the final shape for the profile. The configuration of thedie 60 depends on the desired shape and properties for the resultingprofile. To form hollow profiles, the pultrusion die typically containsa mandrel within its interior so that the fiber material flows betweenthe interior surface of the die and the external surface of the mandrelto form the desired shape. Further, although referred to herein as asingle die, it should be understood that the pultrusion die 60 may beformed from multiple individual dies.

One or multiple layers may be employed for forming the profile. In oneembodiment, for example, multiple layers are employed and initiallyspaced apart from each other in the vertical direction. As they passthrough respective channels of the consolidation die 50, the widths ofthe layers are optionally ribboned to help prevent pressure wedges, andto keep the continuous fibers aligned and twist-free. Although notspecifically shown, a mandrel may also be provided in the interior ofthe consolidation die 50 to help guide the layers into contact with eachother on at least one side of the profile. For example, one side of aprepreg layer and a side of another prepreg layer may be angled so thatthey contact each other and form a side of a hollow profile. The otherside of the profile is, however, typically left open within theconsolidation die 50 so that the long fiber material can be subsequentlyapplied to the interior of the profile in the pultrusion die. When inthe desired position, the prepreg layers are pulled into a pultrusiondie 60 as described above.

If desired, the resulting profile may also be applied with a cappinglayer to enhance the aesthetic appeal of the profile and/or protect itfrom environmental conditions. Referring to FIG. 5, for example, such acapping layer may be applied via an extruder oriented at any desiredangle to introduce a thermoplastic resin into a capping die 72. Theresin may contain any suitable thermoplastic polymer known in the artthat is generally compatible with the thermoplastic polymer used to formthe profile. Suitable capping polymers may include, for instance,acrylic polymers, polyvinyl chloride (PVC), polybutylene terephthalate(PBT), ABS, polyolefins, polyesters, polyacetals, polyamids,polyurethanes, etc. Although the capping resin is generally free offibers, it may nevertheless contain other additives for improving thefinal properties of the profile. Additive materials employed at thisstage may include those that are not suitable for incorporating into thecontinuous fiber or long fiber layers. For instance, it may be desirableto add pigments to the composite structure to reduce finishing labor ofshaped articles, or it may be desirable to add flame retardant agents tothe composite structure to enhance the flame retarding features of theshaped article. Because many additive materials are heat sensitive, anexcessive amount of heat may cause them to decompose and producevolatile gases. Therefore, if a heat sensitive additive material isextruded with an impregnation resin under high heating conditions, theresult may be a complete degradation of the additive material. Additivematerials may include, for instance, mineral reinforcing agents,lubricants, flame retardants, blowing agents, foaming agents,ultraviolet light resistant agents, thermal stabilizers, pigments, andcombinations thereof. Suitable mineral reinforcing agents may include,for instance, calcium carbonate, silica, mica, clays, talc, calciumsilicate, graphite, calcium silicate, alumina trihydrate, bariumferrite, and combinations thereof.

While not shown in detail herein, the capping die 72 may include variousfeatures known in the art to help achieve the desired application of thecapping layer. For instance, the capping die 72 may include an entranceguide that aligns the incoming profile. The capping die may also includea heating mechanism (e.g., heated plate) that pre-heats the profilebefore application of the capping layer to help ensure adequate bonding.

Following optional capping, the shaped part 15 may be supplied to acooling system 80 as is known in the art. The cooling system 80 may, forinstance, be a vacuum sizer that includes one or more blocks (e.g.,aluminum blocks) that completely encapsulate the profile while a vacuumpulls the hot shape out against its walls as it cools. A cooling mediummay be supplied to the sizer, such as air or water, to solidify theprofile in the correct shape.

Following optional capping, the shaped part is then finally cooled usinga cooling system 80 as is known in the art. The cooling system 80 may,for instance, be a vacuum sizer that includes one or more blocks (e.g.,aluminum blocks) that completely encapsulate the profile while a vacuumpulls the hot shape out against its walls as it cools. A cooling mediummay be supplied to the sizer, such as air or water, to solidify theprofile in the correct shape.

Vacuum sizers are typically employed when forming the profile. Even if avacuum sizer is not employed, however, it is generally desired to coolthe profile after it exits the capping die (or the consolidation orcalibration die if capping is not applied). Cooling may occur using anytechnique known in the art, such a vacuum water tank, cool air stream orair jet, cooling jacket, an internal cooling channel, cooling fluidcirculation channels, etc. Regardless, the temperature at which thematerial is cooled is usually controlled to achieve optimal mechanicalproperties, part dimensional tolerances, good processing, and anaesthetically pleasing composite. For instance, if the temperature ofthe cooling station is too high, the material might swell in the tooland interrupt the process. For semi-crystalline materials, too low of atemperature can likewise cause the material to cool down too rapidly andnot allow complete crystallization, thereby jeopardizing the mechanicaland chemical resistance properties of the composite. Multiple coolingdie sections with independent temperature control can be utilized toimpart the optimal balance of processing and performance attributes. Inone particular embodiment, for example, a vacuum water tank is employedthat is kept at a temperature of from about 10° C. to about 50° C., andin some embodiments, from about 15° C. to about 35° C.

As will be appreciated, the temperature of the profile as it advancesthrough any section of the system of the present invention may becontrolled to yield optimal manufacturing and desired final compositeproperties. Any or all of the assembly sections may be temperaturecontrolled utilizing electrical cartridge heaters, circulated fluidcooling, etc., or any other temperature controlling device known tothose skilled in the art.

Referring again to FIG. 5, a pulling device 82 is positioned downstreamfrom the cooling system 80 that pulls the finished profile 16 throughthe system for final sizing of the composite. The pulling device 82 maybe any device capable of pulling the profile through the process systemat a desired rate. Typical pulling devices include, for example,caterpillar pullers and reciprocating pullers. If desired, one or morecalibration dies (not shown) may also be employed. Such dies containopenings that are cut to the exact profile shape, graduated fromoversized at first to the final profile shape. As the profile passestherethrough, any tendency for it to move or sag is counteracted, and itis pushed back (repeatedly) to its correct shape. Once sized, theprofile may be cut to the desired length at a cutting station (notshown), such as with a cut-off saw capable of performing cross-sectionalcuts.

One embodiment of the profile formed from the method described above isshown in more detail in FIG. 11 as element 516. As illustrated, theprofile 516 has a generally rectangular shape and is formed from aribbon 514 within which is distributed a plurality of long fibers 518.The ribbon 514 may be formed from one or more prepregs of the presentinvention. A capping layer 519 also extends around the perimeter of theribbon 514 and defines an external surface of the profile 516. Thethickness of the ribbon may be strategically selected to help achieve aparticular tensile strength and transverse strength (e.g., flexuralmodulus) for the profile. For example, the ribbon may have a thicknessof from about 1.0 to about 4.0 millimeters, and in some embodiments,from about 2.0 to about 3.0 millimeters. The thickness of the cappinglayer 519 depends on the intended function of the part, but is typicallyfrom about 0.1 to about 5 millimeters, and in some embodiments, fromabout 0.2 to about 3 millimeters.

As will be appreciated, the particular profile embodiment describedabove is merely exemplary of the numerous designs that are made possibleby the present invention. Among the various possible profile designs, itshould be understood that additional layers of continuous and/or longfiber material may be employed in addition to those described above.

In addition to pultrusion, other systems may also be employed to processthe prepregs into the desired article. For example, an injection moldingsystem may be employed that includes a mold within which one or moreprepregs may be disposed. The time inside the injector may be controlledand optimized so that thermoplastic resin is not pre-solidified. Whenthe cycle time is reached and the barrel is full for discharge, a pistonmay be used to inject the material to the mold cavity. Compressionmolding systems may also be employed in the present invention. As withinjection molding, the shaping of the prepreg(s) into the desiredarticle also occurs within a mold. The prepreg(s) may be placed into thecompression mold using any known technique, such as by being picked upby an automated robot arm. The temperature of the mold may be maintainedat or above the solidification temperature of the thermoplastic resinfor a desired time period to allow for solidification. The moldedproduct may then be solidified by bringing it to a temperature belowthat of the melting temperature. The resulting product may be de-molded.The cycle time for each molding process may be adjusted to suit theresins used, to achieve sufficient bonding, and to enhance overallprocess productivity.

The mold used in the systems described above may be a single cavity ormulti-cavity mold. The number of the cavities may be determined by theresin used, the cycle time, and the output rate desired. The shape ofthe mold may be selected to produce products for specific applications,such as siding panels, fence picket parts, end caps, joints, hinges,trim boards for interior and exterior decoration, synthetic roofingshingles, slates, shakes or panels, etc.

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is: 1-15. (canceled)
 16. A method for forming athermoplastic prepreg, the method comprising: supplying continuousfibers and long fibers to an extrusion device; supplying a thermoplasticfeedstock to the extrusion device, wherein the feedstock comprises atleast one thermoplastic polymer; and extruding the continuous fibers,long fibers, and the thermoplastic polymer within an impregnation die toform an extrudate in which the continuous fibers are intermixed with thelong fibers and embedded with a matrix of the thermoplastic polymer. 17.The method of claim 16, wherein the feedstock comprises the long fibers.18. The method of claim 16, further comprising pulling the extrudatethrough a nip defined between rollers to consolidate the extrudate intothe form of a sheet.
 19. The method of claim 16, wherein the continuousfibers constitute from about 10 to about 80 wt. % of the prepreg and thelong fibers constitute from about 2 to about 35 wt. % of the prepreg.20. The method of claim 16, wherein the continuous fibers, the longfibers, or both, include glass fibers, carbon fibers, or a combinationof glass and carbon fibers.
 21. The method of claim 16, wherein thethermoplastic polymer includes a polyolefin, polyether ketone,polyetherimide, polyarylene ketone, liquid crystal polymer, polyarylenesulfide, fluoropolymer, polyacetal, polyurethane, polycarbonate,styrenic polymer, polyester, polyamide, or a combination thereof. 22.The method of claim 16, wherein a manifold assembly supplies thethermoplastic feedstock to the extrusion device, the manifold assemblycomprising branched runners through which the thermoplastic feedstockflows.
 23. A method for forming a thermoplastic prepreg, the methodcomprising: supplying continuous fibers to an extrusion device;supplying a thermoplastic feedstock to the extrusion device, wherein thefeedstock comprises at least one thermoplastic polymer; and extrudingthe continuous fibers and the feedstock within an impregnation die toform an extrudate in which the continuous fibers are embedded with amatrix of the thermoplastic polymer; thereafter, applying long fibers tothe extrudate to form a composite.
 24. The method of claim 23, furthercomprising pulling the composite through a nip defined between rollersto consolidate the composite into the form of a sheet.
 25. The method ofclaim 23, wherein the continuous fibers constitute from about 10 toabout 80 wt. % of the prepreg and the long fibers constitute from about2 to about 35 wt. % of the prepreg.
 26. The method of claim 23, whereinthe continuous fibers, the long fibers, or both, include glass fibers,carbon fibers, or a combination of glass and carbon fibers.
 27. Themethod of claim 23, wherein the thermoplastic polymer includes apolyolefin, polyether ketone, polyetherimide, polyarylene ketone, liquidcrystal polymer, polyarylene sulfide, fluoropolymer, polyacetal,polyurethane, polycarbonate, styrenic polymer, polyester, polyamide, ora combination thereof.
 28. The method of claim 23, wherein a manifoldassembly supplies the thermoplastic feedstock to the extrusion device,the manifold assembly comprising branched runners through which thethermoplastic feedstock flows.